Feeding of powders

ABSTRACT

Constant mass-rate feeding of powders is improved by supplying the powder to a feeder in fluid form, and at a uniform bulk density, from a bed of the powder. The depth of the powder bed is maintained essentially constant, thus providing a more constant head of pressure upon the powder being supplied to the feeder.

United States Patent [191 Driscoll [54] FEEDING OF POWDERS [7 r nven or rii s iar E, Drlss alhMonroe L [73] Assignee: Cities Service Company, New York,

[52] US. Cl ..222/l93 [51] Int. Cl. ..B67d 5/54 [58] Field of Search ..222/56, 64, 193,

222/195, 318; 23/1 F; 259/4 D, DIG. 17; 302/53, 57,43, 40,11, 22

[56] References Cited UNITED STATES PATENTS 2,692,798 10/1954 Hicks ..137/563 X 3,219,319 11/1965 Ash ..259/4 3,305,142 2/1967 Caldwell .222/195 [111 3,733,011 11 May 15, 1973 3,307,596 3/1967 Stockel et a1. ..302/53 X 3,223,457 12/1965 Albert ..302/53 2,221,741 1 H1940 Vogel-Jorgensen ..302/53 2,817,310 12/1957 Ponzini ..302/53 X 2,920,635 1/1960 Wilson ..137/563 X 3,333,774 8/1967 Demaison ,302/53 X 3,237,808 3/1966 Witt et a1 ..222/3 1 8 X 3,627,555 12/1971 Driscoll ..222/56 FOREIGN PATENTS OR APPLICATIONS 134,370 11/1919 Great Britain ..302/43 Primary Examiner-Robert B. Reeves Assistant Examiner-Norman L. Stack, Jr. Attorney J. Richard Geaman {57] ABSTRACT Constant mass-rate feeding of powders is improved by supplying the powder to a feeder in fluid form, and at I a uniform bulk density, from a bed of the powder. The depth of the powder bed is maintained essentially constant, thus providing a more constant head of pressure upon the powder being supplied to the feeder.

3 Claims, 1 Drawing Figure FEEDING OF POWDERS This application is a division of United States application Ser. No. 758,880 filed Sept. 10, 1968, now US. Pat. No. 3,627,555 and assigned to the same assignee as this application.

BACKGROUND OF THE INVENTION The term powder" as used herein refers to essentially dry and stable particulate solids which are naturally fluid or which can be rendered fluid by aeration with a gas which is nonreactive with the powder. The term fluid or fluidized as used herein refers to powders which are aerated to the extent that they behave almost as a liquid, e.g. they pour freely without bridging and plugging, conform readily to the outlines of a container, and will of their own accord form into a static bed which is essentially level at the top.

In the processing of powders, it is frequently necessary to provide a stream of the powder which flows at an essentially constant mass-rate. When it is desirable, for instance, to continuously combine the powder with other materials at fixed proportions, it is often times necessary to feed the powder in dry form, and accurate proportioning of the resultant mixture cannot be accomplished unless feeding of the powder to the combining zone is maintained at a constant rate. While suitably accurate constant rate feeding of gases, liquids or dense particulate solids can generally be accomplished without too much difficulty, accurate mass-rate feeding of dry powders has been a long standing problem.-

It has been common practice to supply powder to volumetric displacement or weigh-type feeders from a standing bed of the powder contained within a suitable receptacle. The depth of the bed will fluctuate considerably if the rates at which powder is added to and removed from the bad are significantly unequal. If the powder flows freely and readily conforms to the outline of its container, the bed will have a head pressure which depends upon the depth of the bed, and which will vary proportionately with changes in the bed depth. It has now been discovered that variations in the static head pressure of the powder being supplied to the feeder can cause erratic mass-rate delivery of the powder by the feeder whether the feeder be a volumetric displacement or weigh type.

The constant rate feeding of fluid powder is also adversely affected if the bulk density of the powder intro duced into the feeder is significantly nonuniform. Obviously, mass-rate delivery of powder from a volumetric displacement feeder will fluctuate if the bulk density of the powder supplied thereto is nonuniform, and it has also been observed that delivery from a weigh type feeder may also fluctuate unless the powder is supplied to it at a substantially uniform bulk density, particularly when the powder is in the low bulk density range of about 3 to lbs. per cubic foot.

Problems are also experienced with the feeding of powders which lack or lose fluidity in the powder bed, since the powder may pack or bridge to the extent that free flow to the feeder is interrupted. However, it should be pointed out that highly fluid powders, e.g. powders having a bulk density below about l5 pounds per cubic foot, are particularly difficult to feed at a constant mass-rate, since they are more subject to variation in bulk density and are more difficult to meter accurately, even when the bulk density of the powder remains essentially uniform, unless the powder is supplied to the feeder at a fixed head pressure. Thus, the constant rate feeding of essentially dry powders can be improved by supplying the powder to the feeder in a fluid having an essentially uniform bulk density, and under an essentially constant head pressure.

It is therefore an object of this invention to improve the feeding of a powder whereby the powder may be fed at a more constant mass-rate.

Another object of this invention is to maintain an essentially constant depth in a bed of fluid powder supn plying a powder feeder adapted to deliver the powder at an essentially constant mass-rate.

Still another object of this invention is to supply powder to a powder feeder at a more constant head pressure.

Yet another object is to supply fluid powder to a powder feeder at a more uniform bulk density.

Even another object of the invention is to deliver essentially dry, fluid powder into a combining zone at a constant mass-rate for accurately proportioned combination of the powder with another material.

Other objects and advantages of the invention will become apparent from the following description and the appended claims.

In accordance with the present invention, a bed of fluid powder is formed in a holding receptacle from powder which is continuously supplied to the interior of the receptacle, and the fluid powder is continuously fed from the bed into a suitable feeder through a first powder removal point, the powder removal rate from the bed through this point being less on the average than the rate at which powder is continuously supplied to the bed. However, an essentially constant bed depth is maintained by discharging fluid powder through a second removal point located at a higher elevation in the bed than the first said point. Since the average rate at which powder is supplied to the bed is greater than the average rate at which powder is discharged through the first removal point, the bed depth would continuously increase if it were not for the fact that powder is also removed from the bed through the second removal point located ata higher elevation. In practice, powder is withdrawn from the bed through the second removal point at a rate sufficient to prevent continuous increase of the bed height, so that the depth is maintained and established at a height fixed by the elevation level of the second removal point. More specifically, powder is removed through the second powder removal point at a rate which essentially equals the difference between the total rate at which powder is supplied to the bed and the rate at which powder is discharged through the first removal point. Since the head pressure exerted by the bed at the first removal point varies with the depth of the bed, a more constant head pressure-is thus established by maintaining the bed of powder at a fixed depth.

Many low bulk density powders are highly fluid in the natural state and will remain fluid unless subjected to mechanical compression, agitation, or prolonged settling which results in substantial deaeration. Thus, it is not essential to the invention, inits broadest aspects, that the powder in the bed be subjected to continuous aeration with a nonreactive gas to maintain the powder in a fluid condition and at a uniform bulk density. Since, however,lthe powder of the bed may in some instances be relatively dense, or otherwise subject to packing or bridging due to high head pressure to a tendency to settle rapidly, a process of aeration or reaeration may be employed to maintain the powder fluid and/or to establish uniform density of the powder at the first powder removal point. A nonreactive gas can, for instance, be introduced into the holding receptacle in an amount which renders the powder fluid and maintains a uniform density in the region of the bed just ahead of the first powder removal point. Alternatively, powder may be removed from the receptacle from a locus near the bottom of the bed, agitated in the presence of a nonreactive gas to effect reaeration, and returned to the top of the bed in a fluid condition. If the powder is thus reaerated and recirculated from the bottom to the top of the bed at a rate which approaches or exceeds the natural settling rate of the powder in the bed, fluidity and substantially uniform bulk density can be maintained throughout the bed.

The present invention can be employed to facilitate accurate metering of the powder when it is desirable to treat the powder at fixed mass-rate proportions with a reactive or nonreactive gas or liquid, or when mixing the powder with another solid material.

Thus, the invention can be employed for supplying powder at a constant mass-rate in the following continuous processes:

A. Treatment of the powder particles with a reactive gas or vapor whereby the surface chemical properties of the particles are altered, such as in the treatment of carbon black with oxidizing gases or the treatment of finely-divided silica with steam.

B. Mixture of the powder particles with a nonreactive gas, such as in the formation of aerosols which may be employed, for instance, for conveying the powder pneumatically.

C. Treatment of the powder particles with a reactive liquid whereby the surface chemical properties of the particles are altered, such as in the treatment of carbon black with oxidizing acids or the treatment of finelydivided silica with amines.

D. Dissolution or extraction of the powder particles in a liquid solvent, such as the dissolving of finelydivided silica in hydrofluoric acid or the extraction of powdered coal with liquid hydrocarbons to remove a soluble fraction therefrom.

E. Treatment of the powder particles with a liquid which is essentially nonreactive with the powder particles, and in which the particles are substantially insoluble, such as in the liquid coating of particles, or the aggregation thereof to form pellets, or in the formation of slurries or pastes of the powder.

F. Blending of a powder with other solids, such as in the compounding of rubber with powdered reinforcing agents or fillers.

The invention may also be employed to advantage in a process for modifying the properties of carbon black by attrition of the particles as described in U.S. Pat. No. 3,333,979. As disclosed therein, a stream of carbon black is continuously circulated at a relatively constant mass-rate through an attrition mill. While a feed stream of unmodified carbon black is charged to the mill, a portion of the recirculated stream is withdrawn from the system as treated product. As is illustrated in FIG. 2 of the patent, the degree to which carbon black structure is reduced by the treatment is considerably dependent on the recycle ratio, i.e. the proportion of recirculated black to unmodified black being fed to the attritioning mill. When it is desirable, for instance, to continuously produce a carbon black having its structure reduced to a specific level, it is most important that the proportioning of unmodified feed to recirculated black be maintained essentially constant lest the quality of the product be nonuniform. Since the unmodified black can be fed to the system in the form of relatively dense pellets, i.e. l5-25 lbs. per cubic foot, constant rate feeding of that stream is not too difficult since the pellets flow freely, and the problems of feeding powder which result from variations in head pressure and density are not nearly so serious with pellets. On the other hand, the carbon black which is discharged from the attrition mill is a low bulk density fluid powder, i.e. 3 to 7 lbs. per cubic foot, a sizable portion of which must be recirculated to the mill at a constant mass-rate. By employing the present invention, the recirculation rate to the mill may be more precisely controlled by maintaining the bed of black in the surge tank at aconstant depth, thus passing black from the first removal point to the feeder which controls the recycle rate, and removing black from the second removal point as the product stream. Accordingly, modification of the properties of the carbon black by this attritioning process .may be more precisely controlled by utilizing the present invention in controlling the recirculation rate. It has also been found that use of a feeder for removing a modified-black product stream is not required for maintaining a fixed inventory of black in the system, i.e. the product stream can be obtained by merely allowing the fluid black to overflow from the bed through the second powder removal point.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a somewhat diagrammatical illustration ofapparatus arrangements which may be employed in carrying out the invention.

DESCRIPTION or THE PREFERRED EMBODIMENTS In reference to FIG. 1, an aerosol of powder particles is fed continuously by way ofline 1 into a cyclone separator 2. The aerosol is fractionated within the cyclone 2, so that an aerated, fluid powder is continuously discharged into the receptacle 3 while the depleted gaseous fraction of the aerosol is discharged from the cy clone through line 4. A bed 5 of the fluid powder is thus created within the receptacle 3. Formation of an aerosol for feeding powder to the bed is not essential, however, since if the powder is already available as a fluid, bulk powder it may be added to the bed directly.

In one embodiment of the invention, fluid powder is continuously discharged from the powder bed 5 through an opening 7 in the wall of receptacle 3 and is passed into a feeder, represented at 8, through a conduit 9, which interconnects the feeder with the opening 7. In this instance, the opening 7 is the first removal point through which powder is discharged from the bed. The metered stream of powder which is delivered by the feeder 8 is passed by way of line 10 to any receiving point where constant rate delivery of the powder is desired.

As previously indicated, it is an object of the invention to provide a fixed depth in the powder bed 5 in order to maintain aconstant head pressure on the powder feeder 8. Powder is continuously discharged to the feeder from the bed through a first removal point 7 while powder is continuously added to the bed from cyclone 2 at a rate which, on the average, exceeds the rate at which powder is passed to the feeder. In order to maintain a constant depth in the powder bed 5, it is therefore necessary to remove any excess of powder which might accumulate in the receptacle due to addition of powder to the bed at a faster rate than it is permanently removed therefrom. This is accomplished by removing the excess powder from the bed through conduit 6, which extends vertically into the receptacle, and has an opening 6a in the upper terminus thereof.

It will be noted that opening 6a, which in effect is the second point through which powder is removed from the bed, is located at an elevation significantly higher than the opening 7. Consequently, a positive and essentially constant head of pressure is maintained on the feeder 8 by establishing and maintaining a fixed depth of powder in the bed 5 by removing powder from the bed through the second discharge point at a rate such that the sum of the average rates at which powder is removed from the bed through the first and second re moval points essentially equals the rate at which powder is supplied to the bed. It will be appreciated that with an arrangement as shown in FIG. 1, fluid powder may be discharged through the first and second powder removal points by gravitational flow; i.e. no positive mechanical displacement or suction pressure is required. Powder which is removed through conduit 6 may be passed by way of line 11 to a storage point for subsequent reuse, i.e. it may, for instance, be passed to a surge tank and later reformed into the aerosol which is passed by way of line 1 into the cyclone 2.

In the event that the powder has a tendency to settle rapidly or pack or bridge withing the receptacle 3, it is desirable to preserve a condition of fluidity in the powder bed, especially at the first powder removal point, and this may be accomplished by introducing a nonreactive gas into the powder bed at a locus near the first removal point, the gas being introduced at a rate which aerates the powder and renders it uniformly fluid. Thus, a fluidizing gas may be introduced into the bed through line 12 and subsequently discharged from receptacle 3 through a vent, represented at 13.

Where preferred, the powder of the bed may be maintained fluid and relatively uniform throughout by a process of reaeration and recirculation. For instance, powder may be removed from the bed 5 through conduit l4 and passed by way of a feeder 15 into an agitation zone, represented at 16, through line 14a. A nonreactive gas may be introduced into the agitation zone 16 through line 17 for formation of an aerosol of the powder which is passed by line 18 into cyclone 19. The aerosol may be formed, for instance, by feeding the powder and the nonreactive gas into a fan or blower within which the mixture is vigorously agitated and propelled through line 18 to the cyclone.

The aerosol is fractionated in cyclone l9 and the tea erated, fluid powder returned to the bed 5. Nonreactive gas which is fractionated from the aerosol may be vented or else recycled by way of line 20 to the agitation zone. In practice, the powder is reaerated and re circulated at a rate which equals or exceeds the settling rate of the powder in the bed, and problems of packing, bridging, and nonuniform density are overcome by maintaining the powder highly fluid throughout the bed at all times. Thus, constant mass-rate delivery of powder by the feeder 8 is improved by supplying powder thereto which is uniform in density and under a constant head pressure, since reaeration and circulation of the powder in the bed is carried on in conjunction with maintaining the powder bed at a fixed depth.

When employing the invention in an attritioning pro- 5 cess as described in U.S. Pat. No. 3,333,979, the agitation zone 16 can be the grinding chamber of an attrition mill into which unmodified feed carbon black is introduced at a constant massrate through line 21 while powder is continuously fed from the bed 5 into the ball mill at a constant mass-rate by means of feeder 15. The rate at which carbon black powder is fed from the bed into the mill is at least as great as the rate at which the unmodified black is fed to the mill, and more preferably two to forty times as great. Accordingly, car- [5 through the attrition mill while unmodified feed black is added to the system through line 21. Treated product is removed through conduit 6. in such an operation, the feeder 8 need not be employed, in which case the upper opening of conduit 14 becomes the first powder removal point. Furthermo're, since carbon black is added to the system through line 21, powder need not be fed to the bed through line 1. After passing through the mill the carbon black is circulated back to the bed in fluid form, either as bulk powder or as :an aerosol, as previously described. An essentially constant depth of powder is maintained in bed 5 by continuous overflow of fluid carbon black powder through opening 6a, the average discharge rate from that point being substantially equal to the rate at which carbon black feed is supplied to the attrition mill. It will be appreciated that all carbon black which passes from the attrition mill is recirculated to the powder bed and supplied to it at a total rate which is greater than that at which powder is re moved through the first removal point, thus resulting in product overflow through conduit 6 and a constant depth of powder in the bed 5. The unmodified black may be fed to the attrition mill in powder form, but pellets of the powder can be employed to particular advantage, since they will be disintegrated in the attritioning chamber and the resultant particles subjected to physical modification by attritioning in the chamber. Where desired, a reactive gas may be introduced into the attritioning chamber through line 17 for chemical modification of the carboriblack.

lt should also be pointed out that the recirculating stream of carbon black can be maintained highly fluid and at a substantially uniform density by repeated passes through the mill, thus aerating the black by agitation in the presence of gases which normally exist within the system, e.g. air, thus obviating the need for introducing a gas from an external source for the purpose of aeration. The resultant fluid powder can be conveyed to and from the mill in bulk form by means of elevators, screw conveyors, or the like.

While the invention has been described with particular reference to powdered materials in the submicron particle size range, e.g. carbon black, finely-divided silica, etc., and which are naturally flocculent and characterized by low bulk density, it will be understood that the invention is not limited thereto since it can be employed with any essentially dry and stable particulate solid which exist in a fluid state, or which can be rendered fluid, such as expanded polystyrene beads, vermiculite, or the like.

Therefore what is claimed is:

1. Powder feeding apparatus comprising:

bon black from the bed 5 is constantly recirculated a receptacle for containing powder,

means for supplying powder to a fluid bed of powder contained in the receptacle,

a feeder adapted to deliver powder at a constant mass rate,

means for conveying fluid powder from the receptacle to the powder inlet of the feeder, said conveying means being adapted for discharge of fluid powder from the bed at a first removal point through an opening in the wall of the receptacle, the opening being located near the bottom of the fluid powder bed,

. means for removing fluid powder from the bed in the receptacle at a second point which is at a higher elevation than the first powder removal point,

. means for removing powder from the fluid bed in the receptacle at a third point, means for agitating powder removed at the third point with a nonreactive gas, and means for conveying the thus agitated powder to the top of the fluid powder bed contained in the receptacle.

2. Apparatus of claim 1 in which the means of subparagraph (e) comprises a conduit which intercommunicates with the interior of the powder receptacle, said conduit having an opening adapted for free flow of powder into the conduit, and said opening of the conduit being located at a substantially higher elevation than said powder-discharge outlet which is employed in conveying powder to the feeder.

3. Apparatus of claim 1 including means for introducing a nonreactive gas into the powder bed at a locus near said powder discharge outlet. l Il 

1. Powder feeding apparatus comprising: a. a receptacle for containing powder, b. means for supplying powder to a fluid bed of powder contained in the receptacle, c. a feeder adapted to deliver powder at a constant mass rate, d. means for conveying fluid powder from the receptacle to the powder inlet of the feeder, said conveying means being adapted for discharge of fluid powder from the bed at a first removal point through an opening in the wall of the receptacle, the opening being located near the bottom of the fluid powder bed, e. means for removing fluid powder from the bed in the receptacle at a second point which is at a higher elevation than the first powder removal point, f. means for removing powder from the fluid bed in the receptacle at a third point, means for agitating powder removed at the third point with a nonreactive gas, and means for conveying the thus agitated powder to the top of the fluid powder bed contained in the receptacle.
 2. Apparatus of claim 1 in which the means of subparagraph (e) comprises a conduit which intercommunicates with the interior of the powder receptacle, said conduit having an opening adapted for free flow of powder into the conduit, and said opening of the conduit being located at a substantially higher elevation than said powder-discharge outlet which is employed in conveying powder to the feeder.
 3. Apparatus of claim 1 including means for introducing a nonreactive gas into the powder bed at a locus near said powder discharge outlet. 